Because of concern regarding weight and flexibility, thin film devices including electronic devices such as photovoltaic devices, electrical circuits, electrode structures and the like, as well as non-electronic devices such as filters, catalysts, optical data storage devices and the like, are frequently fabricated on thin, flexible substrates.
The layers of material comprising the active device are frequently deposited by thin film techniques and generally have inherent stresses which cause the thin flexible substrate to curl. Such curling interferes with the use of the device and complicates processing and handling steps. Various techniques have been implemented to eliminate or minimize such curling. In some instances, the material comprising the device is disposed in the form of discrete islands having a relatively small area. However, this approach is not practical for many types of device and requires specialized steps during the deposition process. In other instances, compensating layers having stress in the opposite direction of the direction of curl are deposited onto the front surface of the structure along with the thin film device; however, such layers are not always available, and their disposition can interfere with the function of the device. In other instances, stress compensating layers are deposited on the back side of the substrate opposite the device. While such anti-curl layers are effective, they are not generally compatible with roll-to-roll deposition processes which are frequently employed for the large scale fabrication of thin film ultralight semiconductor devices such as ultralight photovoltaic devices. Such roll-to-roll processes are well known in the art and are disclosed in U.S. Pat. Nos. 4,485,125 and 5,090,356, the disclosures of which are incorporated herein by reference.
In the roll-to-roll fabrication of ultralight, thin film electronic devices such as photovoltaic devices, a thin substrate member is affixed to a carrier member, typically a body of a ferrous alloy material. The carrier member provides mechanical strength and rigidity to the substrate as it is carried through a large scale roll-to-roll processing unit. Furthermore, in some instances the ferrous carrier allows for magnetic handling and guidance of the device during its fabrication and processing. Semiconductor layers are deposited onto the supported substrate, and when device fabrication is finished, the carrier material is removed, typically by etching utilizing an acidic solution such as a solution of ferric chloride. The presence of the carrier member precludes the deposition of a stress compensating layer on the back side of the substrate. The compensating layer could be deposited onto the substrate prior to the time it is affixed to the carrier; however, the thus coated substrate would then tend to curl since stresses generated by the compensating layer are not counterbalanced by stresses generated by the not yet deposited device.
As will be explained in detail hereinbelow, the present invention provides a method and structure which allows stress compensated devices to be fabricated in a roll-to-roll process.